Undefined values are decoded with the specified default value

[ilario-pierbattista]

The usage should be like:

Codecs::classFromArray(
  [
    'foo' => Codecs::string(),
    'bar' => Codecs::union(Codecs::int(), Codecs::undefined(-1))
  ],
  ...
);

Resolves #6

[ilario-pierbattista]

The undefined codec has strange behavior.

In PHP, there is nothing equivalent to the undefined of javascript.
To simulate it I rely on an internal class named Undefined.
The codec will receive a default value by construction, which will substitute this class, as I don't want to expose it.

The encoding process should reverse transform the default value to an instance of the internal class Undefined to make the codec obey the codecs laws (https://github.com/gcanti/io-ts/blob/master/Codec.md#codec-interface).

Therefore this is not a real codec. Maybe something better can be done implementing the invariant mapping (#7)